In eucaryotic organisms, haploid gametes are generated from diploid cells during meiosis. In the first meiotic division, homologous chromosomes migrate to opposite poles of the cell (disjoin). In organisms as diverse as Saccharomyces cerevisiae and Homo sapiens, crossovers between the homologous chromosomes ensure correct meiosis I segregation by holding the pair together until anaphase I when they move apart. An undefined activity, referred to as the chiasma binder, probably enables crossovers to hold homologous chromosomes together. Meiosis is carried out with considerable fidelity and errors are usually lethal. In humans, a number of disorders, including Down's syndrome, are caused by segregation errors in meiosis I. The long term objective of the research described here is to elucidate the mechanisms by which homologous chromosomes are segregated in meiosis I. These experiments will be carried out in the yeast, S. cerevisiae, where it is possible to study meiosis using artificial chromosomes. These model molecules offer the advantages that they mimic the behavior of natural chromosomes but have precisely defined structures and are easily modified to meet specific experimental needs. Most importantly, they carry no essential genes. Therefore, cells which have lost them through meiotic errors are viable and can be analyzed. The experiments in this proposal are designed to accomplish four specific aims. First, these experiments will test the hypothesis that specialized DNA sequence elements are required to mediate homologous pairing and/or crossing-over. Second, they will test the model that intertwining of sister chromatids provides chiasma binder activity. A strategy for identifying sequences with chiasma binder activity will also be presented. A system capable of correctly segregating nonrecombined artificial chromosomes in yeast has recently been described. This system may be utilized to increase the fidelity of meiosis by correctly distributing pairs of natural chromosomes that have failed to recombine. The third goal of these experiments will be to test this hypothesis. The final experimental section describes a strategy for identifying mutants in this system, and cloning the affected genes. The goal here is to acquire a set of mutant strains and cloned genes which can be used as tools to examine how this system functions in yeast and whether it is common to other organisms.